Typical Magnetoresistive random access memory (MRAM) structures have a nonmagnetic layer sandwiched between two ferromagnetic films. The two ferromagnetic films are also known as magnetic thin films. The MRAM employs the magneto resistive properties of this structure to store data. In each storage element, an MRAM employs two lines, commonly termed a word line and a sense string, in order to detect the magnetization direction of these magnetic thin films. Each string comprises a magnetic thin film that serves as a memory element, and the word line generally addresses multiple sense strings. Magnetic thin films that have a parallel moment have a low resistance and are typically assigned the ‘1’ state. Magnetic thin films having an anti-parallel moment have a high resistance and are typically assigned the ‘0’ state, but may also be assigned to the ‘1’ state.
During a read operation, a word current passes through the word line causing the magnetic layers in the sense string to rotate, thereby changing the resistance in the sense string. A sense current passes through the sense string. A sense line receives the signal from the sense string. A differential amplifier compares the signal from the sense line to a reference line to determine whether a one resistance or a zero resistance is stored in the MRAM. A differential amplifier notes the change in voltage across the sense line to determine resistive state of a storage element.
Successful MRAM operation requires that only small variations from desired operation conditions are allowed. Thus, sense current and word current values must be tightly controlled across the chip.
Other solutions involve using a reference voltage that is sent through out the entire chip. There can be a separate reference voltage for the operation of read and another for write. This introduces a number of voltage routing problems. As an example, assume that a signal line is closely routed to a word current line or a sense current line. Closely routed in the sense that the signal line can induce a variation in a voltage signal in the word line or sense line. This variation in voltage is considered noise and interferes with the operation of the MRAM. Attempts to control the voltage accurately fail because of the capacitance between the two closely routed lines. The interfering line could be a signal line or a power supply line that could inject noise into the reference line, which would end up translating into a voltage change on the word line, or sense line, because of a change in charge. This change would be considered noise in the word current source. This can also result from a power supply line running across the sensitive reference lines resulting in capacitance induced voltage changes from charge injection. These coupling mechanisms cause a voltage noise being transferred onto these sensitive reference lines.
There is a need to compensate for these differences in operating conditions.
There is a further need to provide a stable word and sense current.